1. Field of the Invention
The invention relates to silane-modified polyvinyl acetals, to a process for their preparation, and to their use.
2. Background Art
The preparation of polyvinyl acetals, obtained from the corresponding polyvinyl alcohols by polymer-analogous reaction with appropriate aldehydes, was disclosed as early as 1924, and since then a wide variety of aldehydes has been used for preparing the corresponding polyvinyl acetals. Polyvinyl acetals are prepared in a 3-stage process, wherein polyvinyl acetate is hydrolyzed to polyvinyl alcohol which is then converted to polyvinyl acetal. The resultant products contain vinyl alcohol units and vinyl acetate units, as well as vinyl acetal groups. Polyvinyl formal, polyvinyl acetacetal, and polyvinyl butyral (PVB) have achieved particular commercial importance. The term xe2x80x9cmodified polyvinyl acetalsxe2x80x9d hereinafter means polyvinyl acetals which contain other monomer units in addition to vinyl acetate, vinyl alcohol, and vinyl acetal units.
The largest application sector for polyvinyl acetals is the production of safety glass in automotive construction and for architectural uses. For example, plasticized polyvinyl butyral films are used as an intermediate layer in glazing units. Mixtures containing modified polyvinyl butyrals have also been proposed for this purpose, for example those having the acetal units described in EP-A 368832 with sulfonate, carboxylate, or phosphate functionality, which feature improved blocking and flow performance. EP-A 634447 discloses modified polyvinyl butyrals whose main polymer chain contains monomer units which bear sulfonate groups, these polyvinyl butyrals being obtainable via acetalization of the corresponding sulfonate-functional polyvinyl alcohols.
EP-A 461399 discloses polyvinyl butyrals modified with amino groups, used as precipitants. Another application sector for polyvinyl butyrals is their use in anti-corrosion coatings, as is disclosed in EP-A 1055686, for example, where polyvinyl acetals modified with tertiary alkanolamines are employed for this purpose.
Polyvinyl butyrals are also used as binders in paints, and specifically, in printing inks, due principally to their good pigment binding power. One of the requirements of such applications is that organic solutions of the polyvinyl butyrals should have very low solution viscosity, in order to permit their use for producing inks with high solids content, while employing a very high binder content as well. Modified polyvinyl butyrals disclosed in DE-A 19641064 which exhibit low solution viscosity are examples here, and are obtained by acetalizing a copolymer having vinyl alcohol units and 1-alkylvinyl alcohol units.
A disadvantage possessed by all of the polyvinyl acetals described in the prior art is their inadequate adhesion to specific substrates. For this reason, addition of coupling agents is essential in many cases. EP-B 0346768 describes the coating of films or foils with amino-functional silanes, such as N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, in order to improve bonding to other polymers, in particular polyvinyl butyrals. Polyethylene films and polyester films are coated either with vinyltrimethoxysilanes or with chloropropyltri-methoxysilanes in order to improve adhesion, thus providing successful lamination using hot-melt adhesives based on ethylene-vinyl acetate copolymers (E. Plueddemann, xe2x80x9cBonding through Coupling Agentsxe2x80x9d, Plenum Press, New York, 1985). N-2-aminoethyl-3-aminopropyltrimethoxysilane has been employed as a primer coat for improving the adhesion of ionic resins, for example salts of a polymer based on ethylene-methacrylic acid, to glass or polycarbonate films (U.S. Pat. No. 4,663,228). EP-B 0636471 claims a process for producing a glass composite, where the adhesion between glass and polymer film, inter alia polyvinyl butyral, is improved by using a mixture of at least two silanes, one which increases the bond strength between glass and polymer film, while a second silane does not provide bonding between glass and polymer film.
The processes described in the prior art for improving adhesion between polyvinyl acetals and critical substrates by adding known coupling agents have clear disadvantages. For example, the adhesion-promoting action is often not sufficiently long-lasting, or the composite weakens over time. Another disadvantage is the yellowing of aminosilanes, their unpleasant odor, and skin-irritant action, known problems when using amino-functional compounds. Another problem involves the actual addition of the coupling agent, which always requires an additional time-consuming step in the mixing procedure. The selection and addition of suitable coupling agents, furthermore, requires great expertise, since when adhesion promoters are added to polyvinyl acetal in organic solvent, the result can at times be incompatibility and inhomogeneity, extending as far as phase separation. In addition, the reactive silanes subsequently added as coupling agent may become involved in side reactions such as hydrolysis or condensation reactions. A further disadvantage consists in the large amount of adhesion promoter which generally is required in order to achieve noticeably improved adhesion of polyvinyl acetals on critical substrates. As a result, the process can therefore become very expensive, since the price of silanes is high.
Three Japanese publications have disclosed silane-modified polyvinyl acetals. JP-A 06-247760 and JP-A 06-248247 disclose polyvinyl acetals modified with silane-functional monomer units which are used as binders for cementitious compositions or for inorganic fiber materials. These Si-modified polyvinyl acetals are obtained by copolymerizing vinyl acetate with vinylalkoxysilanes, hydrolyzing the vinyl ester-vinylsilane copolymers, and then acetalizing with aldehydes. JP-A 10-237259 discloses a mixture of silane-modified polyvinyl acetal with unmodified polyvinyl acetal for coating materials to be printed using inkjet printers. In the latter reference, vinylester-vinyl-silane copolymers are first hydrolyzed and then mixed with unmodified polyvinyl alcohol for acetalizing with aldehyde. A disadvantage with the procedures found in all these publications is that they always result an polymer in which pure polyvinyl acetal chains are present alongside polyvinyl acetal chains modified with silanol groups, which is the source of the incompatibility. The enrichment of silanol groups in the silane-modified polyvinyl acetal chains also increases the extent of undesired condensation, and thus results in gelling. Only strongly alkaline systems, e.g. cementitious compositions, are capable of breaking down this gel to some extent, but in neutral organic solvents the inhomogeneity and the gel continue to be present, the visible result being marked phase separation, which is disadvantageous.
It was an object of the invention to provide polyvinyl acetals which, even without addition of other materials, exhibit very good adhesion to various substrates, and in particular to the known critical substrates, so that it is possible for the addition of adhesion promoters to be omitted entirely. Surprisingly, it has been found that polyvinyl acetals which are prepared in the manner of the invention, by hydrolyzing and acetalizing polymers containing uniformly distributed vinyl ester and alkoxysilane units prepared by polymerization of vinyl ester and silane monomer wherein the proportions of these monomers are maintained substantially constant during the polymerization (hereinafter termed xe2x80x9csilanized solid resinsxe2x80x9d), have markedly improved adhesion to critical substrates, indeed to the extent that adhesion promoters can be omitted.